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23 September 2014
Recent advances in understanding of meiosis initiation and the apomictic pathway in plants
Chung-Ju R. Wang
 and 
Ching-Chih Tseng
Structure of plant reproductive organs in maize and sequence of events leading to spore or gametophyte formation in anthers and ovules. (A) Longitudinal section of an anther with numerous pollen mother cells (PMCs, shown in gray) that are proliferated from primary sporogenous cells by mitosis, which accompanies the development of surrounding 4 layers of somatic cells. (B) After primary sporogenous cells (PSCs) are differentiated, they first undergo mitotic divisions to produce sporogenous cells (SCs) and further develop into PMCs. By the time when the development of surrounding somatic cells (shown in A) is complete, unknown meiosis initiation siganl is generated to start meisois synchronously in all PMCs of an anther. Each PMC enters meiosis to produce four haploid spore cells. (C) Longitudinal section of an ovule with a single megaspore mother cell (MMC, shown in gray). (D) Schematic illustration showing the sequential development of embryo sac through sexual reproduction or apomictic pathways. In sexual reproduction, the single MMC (shown in gray) is differentiated and then enters meiosis to produce a haploid functional megaspore (FMS), and then develops into an embryo sac. In diplospory apomixis, the specified MMC unergoes an abnormal meiosis or mitosis to produce a diploid FMS. In apospory apomixis, somatic nucellar cells develop into embryo sac without meiosis.

Meiosis, a specialized cell division to produce haploid cells, marks the transition from a sporophytic to a gametophytic generation in the life cycle of plants. In angiosperms, meiosis takes place in sporogenous cells that develop de novo from somatic cells in anthers or ovules. A successful transition from the mitotic cycle to the meiotic program in sporogenous cells is crucial for sexual reproduction. By contrast, when meiosis is bypassed or a mitosis-like division occurs to produce unreduced cells, followed by the development of an embryo sac, clonal seeds can be produced by apomixis, an asexual reproduction pathway found in 400 species of flowering plants. An understanding of the regulation of entry into meiosis and molecular mechanisms of apomictic pathway will provide vital insight into reproduction for plant breeding. Recent findings suggest that AM1/SWI1 may be the key gene for entry into meiosis, and increasing evidence has shown that the apomictic pathway is epigenetically controlled. However, the mechanism for the initiation of meiosis during sexual reproduction or for its omission in the apomictic pathway still remains largely unknown. Here we review the current understanding of meiosis initiation and the apomictic pathway and raised several questions that are awaiting further investigation.

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